It is necessary to cool the material discharged from the combustor to a temperature which permits a good manageability in the discharge system. In plants with a combustor with an air distributing bottom with openings allowing material from the combustion space of the combustor to pass to an ash chamber below this bottom, it is known to supply cooling air to the ash chamber, which cooling air is allowed to rise up through the material and the bottom to the combustion space. In this space, the cooling air is utilized for the combustion. A plant of this kind is disclosed in U.S. patent application Ser. No. 065,475 (Jonsson). The air flow necessary for cooling of material is so large that it is difficult to avoid local fluidization in the ash chamber or in the openings in the air distributing bottom. Such fluidization may prevent the material transport. Cooling by air also involves a risk of sintering, especially if the bed material should contain unburnt fuel.
It is also known to cool the material by means of tubes, placed in the ash chamber, which are traversed by combustion air or water. However, it has been found to be difficult to obtain a sufficient cooling surface within the available space. Pipe installation in the ash chamber is undesirable also from the point of view of service.